Property’s title and deed

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There's this property that is under my sister's and her soon to be ex-husband's name. We're trying to get the mortgage and property only under my sister's and my name since the husband didn't put any funds into the property. What is the best way to avoid legal issues when making decisions for this property and to not let him claim anything when selling the property? (I'm pretty sure he's willing to sign and be cooperative with any procedure)

Thanks for any information!